This invention relates to a sealed gap-less lightning arrester device enclosed in a grounded metal enclosure filled with a pressurized insulating gas and utilizing nonlinear resistors having excellent nonlinear characteristics and a high electrostatic capacity, and more particularly to means for controlling the potential distribution across such a device.
In miniature substations installed in small sites, it is required to utilize small-sized lightning arresters and it has been a common practice to insulate the arresters by utilizing sulfur hexafluoride (SF.sub.6). A plurality of silicon carbide (SiC) elements utilized as nonlinear characteristic elements which are the primary elements of lightning arresters, have been connected in series with a discharge gap therein between an associated high voltage conductor and a grounded metal enclosure filled with pressurized gaseous sulfur hexafluoride, the number of the series connections being determined by the associated system voltage. As a result, for a high system voltage there is the disadvantage that it is difficult to make the resulting lightning small-sized.
Recently, nonlinear resistors of the zinc oxide (ZnO) type have been developed which resistors have the ability to interrupt the power frequency follow current which is produced upon the occurrence of high surge currents at high voltages resulting from lightning strokes etc. and which are not required to be operatively associated with discharge gaps. Those nonlinear resistors have been substituted for silicon carbide elements as above described so as to make it possible to manufacture small-sized lightning arresters more suitable for use in miniature substantions. Such lightning arresters are required only to include a plurality of zinc oxide resistors connected between an associated high voltage conductor and the grounded metal enclosure thereof. The nonlinear resistor formed of the zinc oxide material presents low magnitudes of resistance to high surge currents at high voltages resulting from lightning strokes etc. while presenting very high magnitudes of resistance to currents caused from voltages normally applied thereto so that the nonlinear resistor effectively protects an electric power device from damage due to high surge currents due to lightning strokes etc. However, that nonlinear resistor presents very high magnitudes of resistance to low currents flowing through the system operated in the normal mode so that it functions as an electrostactic capacity rather than a resistance. Accordingly, when operatively coupled to any AC machine, the serially connected nonlinear resistors have different voltage shares in accordance with positions occupied within the associated grounded metal enclosure because of the influence of stray capacities developed between the same and the grounded metal enclosure. As a result, the nonlinear resistors generate unequal amounts of heat and are unevenly deteriorated until they are successively broken. This has resulted in the lightning arresters with a decreased lifetime.
Accordingly, it is an object of the present invention to provide a new and improved enclosed lightning arrester device substantially free from the disadvantages of the prior art devices as above described in which there is provided means for compensating for stray capacities developed between the grounded metal enclosure and a plurality of serially connected nonlinear resistors disposed within the latter so as to make the potential distribution across each of units into which the serially connected nonlinear resistors are equally divided, equal to the potential distributions across the other units.